1. Field of the Invention
The present invention relates to an optical system that is suitable for being applied to optical apparatuses, such as silver halide film cameras, digital still cameras, video cameras, digital video cameras, telescopes, binoculars, projectors, and copiers.
2. Description of the Related Art
It is required that an optical system used for optical apparatuses, such as digital cameras, video cameras, and projectors, is wide-angled, and a high-quality image (a high-quality projected image for a projector) without distortion is obtained.
Wide-angle lenses are suitable for obtaining a natural image without distortion in a wide range on an object side. Normal projection wide-angle lenses satisfy a condition: Y′=f·tan ω, where Y′ denotes the image height, f denotes the focal length of the wide-angle lens (an optical system), and 2ω denotes the entire angle of view (an angle subtended by an object).
In order to effectively obtain a wide angle, normal wide-angle lenses include a negative lens or negative lens unit having a high negative power (refractive power) disposed on an enlargement conjugation side. At that time, in order to obtain further wide-angle lens system, the negative power of the negative lens needs to be increased.
In general, as the power of a negative lens is increased, more negative distortion (distortion aberration) occurs. More distortion of an image occurs in the peripheral portions of the screen, and therefore, it is difficult to maintain the condition: Y′=f·tan ω.
Accordingly, it is desirable that a negative meniscus lens GN having a convex surface on the enlargement conjugation side is disposed furthest away on the enlargement conjugation side. This arrangement allows marginal rays (light rays having marginal angles) to be made incident at an angle close to the normal to each surface portion. Accordingly, the occurrence of distortion can be easily prevented.
An example of a wide-angle optical system having such an arrangement is described in U.S. Pat. No. 6,791,765.
In general, as the angle of view increases, the lens open angle of the negative lens GN tends to increase.
Japanese Patent Laid-Open No. 2007-094174 and Japanese Patent Laid-Open No. 2005-173275 describe a super wide-angle lens having an entire view angle (2ω80°) and including the negative lens GN having a significantly large lens open angle.
As the open angle of the lens is increased so that the shape of the lens becomes closer to being hemispherical (θ=90°), the dimensional accuracy of a polished surface is decreased. In addition, an anti-reflection evaporated film is not uniformly applied to the peripheral portions of the surface. Consequently, the performance of the film is significantly decreased, and therefore, ghost may occur.
In contrast, a wide-angle optical system is proposed in which an aspherical surface is disposed in the vicinity of a negative lens disposed furthest away on the enlargement conjugation side so that the distortion is sufficiently corrected and the open angle of the negative lens is slightly decreased for a wide-angle lens (refer to, for example, U.S. Pat. No. 6,621,645 or U.S. Pat. No. 6,687,061).
As the angle of view of an optical system is increased, the open angle of the surface of a negative lens on the reduction conjugation side disposed furthest away on the enlargement conjugation side is increased. At that time, if the open angle of the surface of a negative lens on the reduction conjugation side is decreased, chromatic aberration of magnification in a short wavelength range significantly occurs.
For example, in general, a wide-angle lens obtains a viewing angle by using a negative lens disposed furthest away on the enlargement conjugation side (a first negative lens). In addition, the wide-angle lens uses a material having a high refractive index and a low dispersion for the negative lens in order to prevent overcorrection of first-order chromatic aberration of magnification due to the high negative power (refractive power).
Nowadays, many optical materials having high refractive index and a low dispersion have second-order dispersion (dispersion in a short wavelength range).
For this reason, if the open angle of the surface of the negative lens on the reduction conjugation side is slightly decreased for the wide angle of view, the refractive power of the surface of the negative lens on the enlargement conjugation side is increased. Accordingly, chromatic aberration of magnification in a short wavelength range significantly occurs on the surface on the enlargement conjugation side.
As a result, for wide-angle lenses having a viewing angle 80 degrees or more, it is significantly difficult to correct chromatic aberration of magnification over a wide wavelength range while decreasing the open angle of a surface of a negative lens on the reduction conjugation side.
Therefore, for existing wide-angle lenses, it is required to sufficiently correct distortion and chromatic aberration of magnification in a short wavelength range while decreasing the open angle of the surface of a negative lens on the reduction conjugation side disposed furthest away on the enlargement conjugation side.